EP-A-0 835 882 describes a process and an apparatus for the simultaneous milling and drying of moist cellulose ether. In the case of the process and the apparatus, a gas stream is divided into three partial gas streams. Although the gas stream is optionally recycled, EP-A-0 835 882 gives no instruction as to how overheating can be counteracted in the case of the circulation gas procedure. Furthermore, a condenser is used in the circulation in the circulation gas procedure, so that steam cannot be used as a carrier gas.
EP-A-0 227008 describes a process and a device for operating a milling plant. The application relates to a process for operating a milling plant through which the product and a carrier gas flows, in which process a circulated inert gas is used as the carrier gas. In the case of this process, only the milling of a material to be milled is described. Combined milling and drying by a superheated carrier gas is not envisaged.
WO 98/31710 describes a mill-dryer for the preparation of polysaccharide derivative powders, dissolved or swollen polysaccharide derivatives being used as the material to be milled. The dissolved or swollen polysaccharide derivative is comminuted by repeated impact and/or shear stress between rotating and stationary or counter-rotating grinding tools and/or by impact of a plurality of particles. At the same time, the water present in the solid is evaporated. The thermal energy required for this purpose is only partly introduced by superheated steam. The electrical energy of the mill drive, which is converted into heat by friction, simultaneously contributes towards the drying. The finely divided solid particles are separated from the gas stream in a separator downstream of the mill-dryer. Said separator may be a centrifugal separator, such as, for example, a cyclone, or a filter separator. Depending on the mill design, classification by screening can already be effected internally. Any granular fraction present is separated from the fine material as a result of the centrifugal force predominating over the drag forces of the transport gas. The fine material is discharged as finished product from the milling chamber with the transport gas. The granules are recycled to the milling zone in an internal or external granule recycling. Alternatively or in addition, it may be expedient to install a further downstream classification stage involving screening or preferably sieving. The coarse fraction separated off there can optionally be recycled to the mill or mixed with the feed.
The temperature of the steam required for combined milling and drying by means of superheated steam is chosen so that the temperature does not fall below the dew point anywhere, depending on the chosen product throughput and/or the amount of water in the polysaccharide derivative.
By using water in the combined milling and drying by means of superheated steam, an oxygen-poor atmosphere is achieved inside the mill-dryer.
In a variant of the process, superheated steam is circulated and the excess superheated steam is discharged as a partial steam stream. A heat exchanger which heats the circulation gas stream to the temperature required for the combined milling and drying is installed before the mill-dryer.
In practice, a circulation gas procedure proves problematic since there is no longer any consumer for the thermal energy in the event of the absence of or a sudden drop in the stream of material to be milled. Even if the heating power of the heat exchanger is spontaneously minimized, such as, for example, by extinguishing the burner flame, the energy stored in the heat exchanger is sufficient for substantially heating up the total circulation of the combined milling and drying. This heating-up continues to such an extent that product adhesions in the mill or residual products in the cyclone or filter separator decompose and the product quality of the subsequently produced product is reduced. Furthermore, in the event of strong heating of the filter materials, fires or at least melting of the filter may occur, with considerable material damage.
In order to avoid such disadvantages, according to the prior art a cooling medium which acts as a consumer in place of the material to be milled is used. Such a consumer may be, for example, air or nitrogen. Another possibility consists, for example, of spraying in water directly behind the heat exchanger in order to consume the excess energy through evaporation. Various disadvantages are associated with these cooling methods; thus, with the use of air, displacement of the steam atmosphere in the mill occurs and this may result in oxidation of product adhesions and residual product. In the extreme case, fires and explosions may result.
The use of nitrogen as a coolant is very expensive and large amounts of nitrogen may constitute a considerable safety risk for people (e.g., asphyxiation due to lack of oxygen in the event of leaks).
Furthermore, gases such as air or nitrogen have a low heat capacity so that the cooling effect is only small.
Spraying in water is technically very complicated; large amounts of water have to be sprayed and it is necessary to ensure that pools of water do not occur in the plant, subsequently leading to considerable product adhesions. In order to avoid incrustations on the nozzles due to salts contained in the water, demineralized water has to be used.
Common to all cooling methods is the fact that they are uneconomical since the energy stored in the plant is discharged unused and has to be reintroduced on resumption of production.
Another aspect common to all cooling methods is that, when the stream of material to be milled is still present but greatly reduced, they intervene significantly in the result of the combined milling and drying by changing the composition of the circulation gas and hence adversely affect the product quality.